Having short development cycles, logic circuits that can reconfigure functions according to programs have been used for mobile devices such as portable telephone devices. Also, such logic circuits have been studied as the key devices of information systems with reprogrammable hardware. Reconfigurable logic circuits include circuits having FPGAs (Field Programmable Logic Arrays) of the LUT (Look Up Table) type or vMOS circuits. However, those circuits have the problems of being volatile and having a large number of elements.
To solve those problems, a circuit including spin transistors having spin-dependent conductivity, such as spin MOSFETs, is disclosed in Non-Patent Reference 1. The structure of a spin MOSFET is shown in FIG. 1(a) of Non-Patent Reference 1. A spin MOSFET has the same structure as a MOSFET, except that the source and the drain contain a conductive ferromagnetic material. In a case where the magnetization directions of the source and the drain are parallel to each other (parallel arrangement), the output current is different from the output current observed in a case where the magnetization directions of the source and the drain are antiparallel to each other (antiparallel arrangement) (FIG. 1(b) of Non-Patent Reference 1).
A logic circuit that utilizes the variable conductance by changing the magnetization directions to control the logic threshold value of an inverter circuit is disclosed (FIG. 8 of Non-Patent Reference 2: Conventional Technique 1). This inverter circuit has a spin MOSFET as one of the FETs. The magnetization arrangement of the spin MOSFET is switched between the parallel arrangement and the antiparallel arrangement, so as to control the logic threshold value of the inverter circuit.
A logic circuit that has an AND circuit function and an OR circuit function by changing the magnetization arrangement of the spin MOSFET with the use of the above inverter circuit is also disclosed (FIG. 2(a) of Non-Patent Reference 1: Conventional Technique 2). Further, a logic circuit that has all-symmetric Boolean functions (AND, OR, XOR, NAND, NOR, XNOR, ALL0, and ALL1) with the use of four spin MOSFETs and three inverter circuits is disclosed (FIG. 4(a) of Non-Patent Reference 1: Conventional Technique 3).
In this specification, the low level of a logic circuit is represented by “0”, the high level is represented by “1”, and the intermediate level between the low level and the high level is represented by “0.5”. “Analog inputs” are inputs of the intermediate level “0.5 ” as well as the inputs “0” and “1” to be input to a logic circuit (or an inverter circuit, for example), in response to a two-input “0” or “1” that is input to input terminals of the logic circuit. Also, “weighting of an analog input” is the rate at which the input of an input terminal is input to a transistor in a case where the logic circuit includes two or more transistors connected to the input terminals. By each conventional technique, a floating gate formed with a capacitance is used to make an analog input to the logic circuit of the conventional technique from a two-input input terminal. “Reconfiguring a logic circuit function in a nonvolatile manner” is being able to change the logic threshold value in a nonvolatile manner in a case of an inverter circuit, for example, and is being able to switch between an AND circuit function and an OR circuit function in a nonvolatile manner in a case of a Boolean logic circuit, for example, where the same circuits are used.
[Non-Patent Reference 1]
Tomohiro Matsuno, Satoshi Sugahara, and Masaaki Tanaka, “Novel Reconfigurable Logic Gates Using Spin Metal-Oxide-Semiconductor Field-Effect Transistors”, Japanese Journal of Applied Physics, Japan Society of Applied Physics, 2004, Vol. 43, No. 9A, p.p. 6032-6037
[Non-Patent Reference 2]
Masaaki Tanaka, “Semiconductor Spin Electronics”, Applied Physics, Japan Society of Applied Physics, 2004, Vol. 73, No. 4, p.p. 514-515
A spin MOSFET, however, is based on a MOSFET and involves a large charge amount. Therefore, to make an analog input to a logic circuit including a spin MOSFET, it is necessary to use a large-area floating gate. For example, it is necessary to prepare a floating gate having an area 100 or more times as large as the area of one MOSFET. As a result, the area of the logic circuit becomes large. Moreover, since a spin MOSFET is based on a MOSFET, the current consumption and the area of each transistor are large.